There is an increasing demand of wideband antennas in wireless communication devices, in order to allow communication in several frequency bands and for different systems. Ultra Wide Band (UWB) signals are generally defined as signals having a large relative bandwidth (bandwidth divided by carrier frequency) or a large absolute bandwidth. The expression UWB is particularly used for the frequency band 3.2-10.6 GHz, but also for other and wider frequency bands.
The use of wideband signals is associated with many positive aspects and advantages as for example described in “History and applications of UWB”, y M. Z. Win et. al, Proceedings of the IEEE, vol. 97, No. 2, p. 198-204, February 2009.
Another important aspect of the UWB-technology is that it is a low cost technology. Recent development of CMOS processors transmitting and receiving UWB-signals has opened up for a large field of different applications and they can be fabricated at a very low cost for UWB-signals without requiring any hardware for mixers, RF (Radio Frequency)-oscillators or PLLs (Phase Locked Loops).
UWB technology can be implemented in a wide range of areas, for different applications, such as for example short range communication (less than 10 m) with extremely high data rates (up to or above 500 Mbps), e.g. for wireless USB similar communication between components in entertainment systems such as DVD players, TV and similar; in sensor networks where low data rate communication is combined with precise ranging and geolocation, and radar systems with extremely high spatial resolution and obstacle penetration capabilities, and generally for wireless communication devices.
It is challenging to generate, transmit, receive and process UWB signals, since it requires the development of new techniques and arrangements within the fields of generation of signals, signal transmission, signal propagation, signal processing and system architectures.
Basically UWB antennas can be divided into four different categories. The first category comprises a so called scaled category, comprising bow-tie dipoles, see for example “A modified Bow-Tie antenna for improved pulse radiation”, by Lestari et. al, IEEE Trans. Antennas Propag., Vol. 58, No. 7, pp. 2184-2192, July 2010, biconical dipoles as for example discussed in “Miniaturization of the biconical Antenna for ultra wideband applications” by A. K. Amert et. al, IEEE Trans. Antennas Propag., Vol. 57, No. 12, pp. 3728-3735, December 2009.
The second category comprises so called self-complementary structures as e.g. described in “Self-complementary antennas” by Y. Mushiake, IEEE Antennas Propag. Mag., vol. 34, No. 6, pp. 23-29, December 1992. The third category comprises travelling wave structure antennas, e.g. the so called Vivaldi antenna which is a well known and widely used antenna, as e.g. discussed in “The Vivaldi aerial” by P. J. Gibson, Proc. 9th European Microwave conference, pp. 101-105, 1979. The fourth category comprises multiple resonance antennas like log-periodic dipole antenna arrays.
Antennas from the scaled category, the self-complementary category and the multiple reflection category comprise compact, low profile antennas with low gain, i.e. having wide and often more or less omni-directional far field patterns, whereas antennas of the travelling wave category, like the Vivaldi antennas, are directional.
The above-mentioned UWB antennas were mainly designed for use in normal Line-of-Sight (LOS) antenna systems with one port per polarization and a known direction of the single wave between the transmitting and receiving side of the communication system.
However, most environments have many objects (such as houses, trees, vehicles, humans) between the transmitting and receiving sides of the communication systems that cause reflections and scattering of the waves, resulting in a multiple of incoming waves on the receiving side. Interference between these waves causes large level variations known as fading of the received voltage (known as the channel) at the port of the receiving antenna. This fading can be counteracted in modern digital communication systems that make use of multiport antennas and support MIMO technology (multiple-input multiple-output). However, so far, there exists no wideband multiport antenna suitable for such MIMO communication systems.
Future wireless communication systems are supposed to comprise a large number of micro base stations with multiband multiport antennas enabling MIMO. Known solutions do not meet requirements as to compactness, angular coverage, radiation efficiency and polarization schemes, which all are critical issues for the performance of such systems. The radiation efficiency of a multiport antenna is reduced by ohmic losses and impedance mismatch like in single-port antennas, but also by mutual coupling between the antenna ports. Therefore, this mutual coupling should be low, but there is not known any compact multiport antenna with low mutual coupling between the ports.
The bow-tie antenna described in SE 535 251 is a single port directional UWB antenna and does not solve the problems referred to above.